1. Field of the Invention
This invention relates to superconductor integrated circuits, and in particular provides a non-carbon containing etchant for the etching of insulators in superconductor integrated circuits.
2. Description of Related Art
Superconductor integrated circuits are potentially valuable components of future radar systems, infrared imaging systems, and satellite communications systems. Superconductive electronics offers high speed, low noise, and low power dissipation.
Superconductor integrated circuits using Josephson tunnel junction devices have been described in a number of U.S. patents including 4,430,662 issued Feb. 7, 1984, and 4,554,567 issued Nov. 19, 1985, and 4,498,228, issued Feb. 12, 1985, all to Jillie and Smith, and all describing integrated circuitry and methods of manufacture. Josephson junction device configurations are also shown in U.S. Pat. Nos. 4,423,430 issued Dec. 27, 1983 to Hasuo et al. and 4,421,785 issued Dec. 20, 1983 to Kroger and U.S. Pat. No. 4,609,903 issued Sept. 2, 1986 to Toyokura et al. A paper by S. Kosaka et al. entitled "An Integration of All Refractor Josephson Logic LSI Circuit", (IEEE Transactions On Magnetics, Volume MAG-21, No. 2, March 1985, pp. 102-109) provides several examples of etch stop by chemical selectivity, (i.e. the layer being etched has a much higher etch rate than the layers underneath).
The fabrication of refractory superconducting integrated circuits is facilitated by the use of reactive ion etching. The material constituents used in these circuits includes Mo, Nb, NbN, and Ti and the dielectrics SiO.sub.2 and Al.sub.2 O.sub.3. These materials, especially the refractory metallics, present great difficulty in obtaining even nominal feature resolution of 1.0 micron using lift off and wet etching techniques. A reactive ion etching (RIE) based process allows superior pattern transfer and improved edge delineation compared to wet etching methods. RIE allows fairly uniform etching over the entire surface with well defined profile and step characteristics compared to other transfer methods. It is also possible by proper selection of the etching gas and conditions to obtain pronounced etch selectivity, i.e. a metallic constituent may be etched preferentially with respect to the dielectric or the dielectric may be preferentially removed leaving the metallic relatively unaffected. Photoresist masks are generally used in addition to RIE selectivity to define the circuit components.
The etchant gas and plasma conditions must be carefully determined to achieve an acceptable etch selectivity. Optimization to obtain maximum selectivity is determined by detailed experimental testing to establish proper mixtures and gas ratios, flow rates and plasma electrical parameters. The goal of the optimization process is to obtain the greatest ratio between the etch rate of the overlayer and the etch rate of the underlayer.
Selectivity can be demonstrated, for example, by comparing the etching of a Nb and Al.sub.2 O.sub.3 (or Al) two layer composite with CF.sub.4 vs. CCl.sub.4 vs. CCl.sub.4. Al.sub.2 O.sub.3 or Al can be easily etched by using a chlorine containing gas such as CCl.sub.4. The reacted product is then AlCl.sub.3 which has a significant vapor pressure at a relatively low temperature (.about.1.0 torr at 100.degree. C.) and is thus easily dissociated by RIE at ambient temperatures. It is difficult to etch Al.sub.2 O.sub.3 preferentially in a circuit configuration where Nb is also present and unprotected, unless the Nb is attacked at a markedly reduced rate. That result, in fact, is obtained for CCl.sub.4, where Al.sub.2 O.sub.3 (or Al) etches faster than Nb, providing modest selectivity for Al.sub.2 O.sub.3 over Nb. By changing the etch gas to CF.sub.4, the etch rate for Nb will remain almost the same as the etch gas to CF.sub.4, the etch rate for Nb will remain almost the same as observed for CCl.sub.4, with virtually no etching seen for the Al.sub.2 O.sub.3. Etching Al.sub.2 O.sub.3 with CF.sub.4 forms AlF.sub.3, which has a significant vapor pressure only at very high temperature (.about.1.0 torr at 1238.degree. C.) and thus almost impossible to dissociate or etch in an RIE process. It is thus possible with CF.sub.4 to easily selectively etch Nb leaving Al.sub.2 O.sub.3 completely unaffected.
Selectivity of etch rates is a desired condition, but other factors may also be important. Problems are encountered when one attempts to etch vias through a layer of SiO.sub.2 insulator down to a metallic underlayer of Nb or NbN. The standard techniques to reactive ion etch the SiO.sub.2 is to use CHF.sub.3 as noted by M. Radparvar et al., ("Fabrication and Performance of All NbN Josephson Junction Circuits," IEEE Trans. Mag., Vol. MAG-23, No. 2, pp. 1480-1483, March 1987) and L. S. Yu et al. ("An All-Niobium Eight Level Process for Small and Medium Scale Applications," IEEE Trans. on Magnetics, Vol. MAG-23. No. 2, pp. 1476-1479, March 1987), or to use CHF.sub.3 and O.sub.2 as noted by S. Kotani et al. "High-Speed Unit-Cell for Josephson LSI Circuits Using Nb/AIOx/Nb Junctions," IEEE Trans. on Magnetics, Vol. Mag-23, No. 2, pp. 869-874, March, 1987).